The present invention relates to a method for producing a bonding wafer by the so-called hydrogen ion delamination method (also called a smart cut method) comprising bonding an ion-implanted wafer to another wafer that serves as a substrate and then delaminating the wafers, which method can reduce failures to be generated in a peripheral portion of the wafer after the delamination.
As a method for producing an SOI (Silicon On Insulator) wafer using the so-called bonding method, there is known a technique comprising bonding two of silicon wafers via a silicon oxide film, for example, a method comprising forming an oxide film on at least one of such wafers, bonding the wafers to each other without interposing foreign matters between the surfaces to be bonded, and then subjecting them to a heat treatment at a temperature of 200-1200xc2x0 C. to enhance the bonding strength, as disclosed in Japanese Patent Publication (Kokoku) No. 5-46086.
The bonding wafer, of which bonding strength was enhanced by such a heat treatment, can be subjected to subsequent grinding and polishing processes. Therefore, an SOI layer for fabricating elements can be formed by reducing thickness of the wafer on which elements are to be fabricated (bond wafer) to a desired thickness by grinding and polishing.
A bonding SOI wafer produced as described above has advantages of superior crystallinity of SOI layer and high reliability of buried oxide layer existing directly under the SOI layer. However, because it is produced through reduction of thickness by grinding and polishing, the reduction of thickness takes a lot of time and generates waste of the material. In addition, obtainable uniformity of the thickness is only in such a degree of target thickness xc2x10.3 xcexcm at most.
Further, since there are portions called polishing sag at the peripheral portions of two mirror-surface wafers to be bonded, the portions cannot be bonded and they are left as unbonded portions. If the thickness reduction is performed with existence of such unbonded portions, failures such as delamination of the unbonded portions may be caused during the thickness reduction process. Therefore, these unbonded portions must be removed beforehand (see, for example, Japanese Patent Laid-open (Kokai) Publication No. 3-250616).
Meanwhile, in connection with recent use of higher integration degree and higher processing velocity of semiconductor devices, further reduction to thin film thickness and improvement of film thickness uniformity are required as for the thickness of the SOI layer. Specifically, a film thickness and uniformity represented as 0.1xc2x10.01 xcexcm or so are required.
Because a thin film SOI wafer having such a film thickness and film thickness uniformity cannot be realized from a bonding wafer by the conventional thickness reduction processing through grinding and polishing, the method called hydrogen ion delamination method was developed as a novel film thickness reduction technique as disclosed in Japanese Patent Laid-open (Kokai) Publication No. 5-211128.
This hydrogen ion delamination method is a technique for producing an SOI wafer, wherein an oxide film is formed on at least one of two silicon wafers, hydrogen ions or rare gas ions are implanted into one wafer (also referred to as bond wafer hereinafter) from its top surface to form a micro bubble layer (enclosed layer) in this silicon wafer, then the ion-implanted surface of the wafer is bonded to the other wafer (also referred to as base wafer hereinafter) via the oxide layer, thereafter the bond wafer is delaminated at the micro bubble layer as a cleavage plane (delaminating plane) by a heat treatment (delamination heat treatment), and the base wafer on which a silicon layer is formed as a thin film (SOI layer) is further subjected to a heat treatment (bonding heat treatment) to strengthen the bonding to obtain an SOI wafer.
In this hydrogen ion delamination method, it is also possible to directly bond silicon wafers to each other without an oxide film after the ion implantation, and it can be used not only for a case where silicon wafers are bonded to each other, but also for a case where an ion-implanted silicon wafer is bonded to an insulator wafer having a different thermal expansion coefficient such as those of quartz, silicon carbide, alumina and so forth.
By using the hydrogen ion delamination method, the delaminated plane can be obtained as a good mirror surface. Therefore, for example, when an SOI wafer is produced, an SOI wafer having an extremely high uniformity of the SOI layer can be relatively easily obtained. In addition, since the bond wafer after the delamination (also called delaminated wafer hereinafter) can be recycled, the method enjoys an advantage that the material can be used effectively.
Further, since the unbonded peripheral portions are left on the delaminated wafer upon the delamination, it also has an advantage that such a process of removing the unbonded portions of peripheral portions of wafers as disclosed in the aforementioned Japanese Patent Laid-open (Kokai) Publication No. 3-250610 becomes unnecessary. This is one of the important advantages of the hydrogen ion delamination method including the obtainable film thickness uniformity of SOI layer and the possibility of recycling of the material.
By actually observing a peripheral portion of SOI wafer produced by the hydrogen ion delamination method, it can be seen that the peripheral end of the SOI layer locates in a inside region of about 1 mm from the peripheral end of the base wafer. This is because portions of about 1 mm from the peripheral ends of the bonded two wafers are not bonded due to the polishing sag of the peripheral portions thereof and hence delaminated.
The width of the unbonded portions from the peripheral ends depends on size of the polishing sag, and it is known that it is usually about 1 mm or about 2 mm at most when a usual mirror polished silicon wafer is used.
However, it has become clear that, if an SOI wafer produced by the hydrogen ion delamination method as described above is subjected to various processes such as heat treatment, cleaning and device production, problems may occur including generation of particles from peripheral portion of the wafer, generation of cracks in the SOI layer and so forth, although occurring frequency is not so high. Because generation of such particles, cracks and so forth may cause reduction of yield or degradation of characteristics in the device production process using SOI wafers, it must be avoided as much as possible.
The cause of the aforementioned generation of particles, cracks and so on is considered as follows. That is, peripheral portions of a bonding wafer produced by the hydrogen ion delamination method do not have unbonded portions and they are physically bonded. However, their bonding strength is not necessarily sufficient compared with the wafer center portions due to the sag in the wafer pheripheral portions generated before the bonding. It is considered that, therefore, particles are generated or cracks are formed in the SOI layer from such wafer peripheral portions having insufficient bonding strength during the various heat treatment processes, cleaning process, device production process and so forth after the delamination.
Such a problem is not limited to SOI wafers formed by bonding silicon wafers to each other via an oxide film, but commonly observed in all bonding wafers produced by the hydrogen ion delamination method, for example, the aforementioned SOI wafers utilizing insulator wafers such as those of quartz, silicon carbide, alumina and so forth as the base wafer, or bonding wafers formed by directly bonding silicon wafers without an oxide film.
In view of the above problems, an object of the present invention is, when producing a bonding wafer by the hydrogen ion delamination method, to produce a bonding wafer free from the problems of the generation of particles from the peripheral portion of the wafer and the generation of cracks in the SOI layer and so forth.
In order to achieve the aforementioned object, the present invention provides a method for producing a bonding wafer by the hydrogen ion delamination method comprising at least a step of bonding a base wafer and a bond wafer having a micro bubble layer formed by gas ion implantation and a step of delaminating them at the micro bubble layer as a border, wherein a peripheral portion of a thin film formed on the base wafer is removed after the delamination step.
By removing a peripheral portion having insufficient bonding strength of the thin film formed on the base wafer after the delamination step in a method for producing a bonding wafer by the hydrogen ion delamination method as described above, a bonding wafer having sufficient bonding strength over the whole bonded area can be provided, and thus the problems in a device production process and the like, which are the generation of particles from the peripheral portion of the thin film, the generation of cracks in the thin film and so forth, can be prevented.
The present invention further provides the aforementioned method for producing a bonding wafer in which the thin film has at least an SOI layer.
When the wafer is a bonding SOI wafer in which the thin film formed on the base wafer consists of an SOI layer, or an SOI layer and an insulating film such as an oxide film, at least a peripheral portion of the SOI layer formed on the base wafer can be removed after the delamination step to provide an SOI wafer free from the generation of particles from the peripheral portion of the wafer and the generation of cracks in the SOI layer.
When the peripheral portion of the thin film is removed as described above, it is preferable to remove a region of 1-5 mm from the peripheral end of the base wafer.
Further, when the wafer is a bonding SOI wafer in which the thin film formed on the base wafer has at least an SOI layer, the removal of the peripheral portion of the thin film is preferably attained by removing at least the SOI layer for a region of 1-5 mm from the peripheral end of the base wafer.
Since the portion showing insufficient bonding strength with respect to the base wafer in the thin film usually exists in such a region, the portion having insufficient bonding strength can be surely removed by forcibly removing that portion of the peripheral portion of the thin film, and thus a bonding wafer in which the whole thin film is firmly bonded to the base wafer can be obtained.
The removal of the peripheral portion of the thin film can be attained by etching the wafer with masking at least portions of the top surface other than the peripheral portion to be removed.
By performing the etching in such a manner as described above, the peripheral portion of the thin film can be easily and surely removed.
Further, as another method, the peripheral portion of the thin film can also be removed by holding together a plurality of wafers stacked so that at least the peripheral portions to be removed should be exposed, and etching them. By such a method, many wafers can be etched simultaneously, and hence the peripheral portion of the thin film can be removed efficiently.
Furthermore, the removal of the peripheral portion of the thin film can be attained by polishing only the peripheral portion. The peripheral portion of the thin film can also be easily and surely removed by such polishing of only the peripheral portion.
The present invention further provides a bonding wafer produced by the hydrogen ion delamination method, wherein a thin film formed on a base wafer is removed for a region of 1-5 mm from a peripheral end of the base wafer.
Such a bonding wafer can be produced by the aforementioned production method of the present invention, and is free from the problems of the generation of particles from the peripheral portion of the wafer, the generation of cracks in the thin film and so forth.
The present invention also provides the aforementioned bonding wafer, wherein the thin film has an SOI layer and at least the SOI layer is removed for a region of 1-5 mm from the peripheral end of the base wafer.
Such a bonding SOI wafer in which the peripheral portion of the SOI layer is removed for a region of the range from the peripheral end of the base wafer defined above has a film thickness and film thickness uniformity suitable for recent semiconductor devices of high integration degree and high processing velocity. In addition, since the SOI layer is bonded to the base wafer with sufficient bonding strength over the whole wafer, the problems of the generation of particles from the peripheral portion of the wafer and the generation of cracks in the SOI layer during the device production process and so forth are substantially avoided.
As explained above, according to the present invention, when a bonding wafer is produced by the hydrogen ion delamination method, a peripheral portion of thin film such as SOI layer formed on the base wafer is removed after the delamination step, and thereby a region of insufficient bonding strength can be eliminated and there can be provided a bonding wafer having sufficient bonding strength for the whole wafer.
Such a bonding wafer does not substantially generate particles from the peripheral portion of the thin film and so forth or does not generate cracks in the thin film during subsequent cleaning process, device production process or the like. Therefore, it has advantages of marked reduction of characteristics degradation and improvement of yield.